A flat-plate collector gets incident of solar radiation at a rate of 260 Btu/h-ft' and has the collector properties as shown. If the collector has an average temperature of 120°F and ambient air temperature is 67°F, determine the ratio (%) of efficiency difference between the single glazing collector and the double glazing collector to the efficiency of the single glazing collector. Typical flat-plate solar collector properties U, W/m2C U, Btu/h-t2F Ta No gazing Single glazing Double glazing 0.90 0.85 0.80 2.8 0.5 1.7 0.3 90 00 221

A flat-plate collector gets incident of solar radiation at a rate of 260 Btu/h-ft' and has the collector properties as shown. If the collector has an average temperature of 120°F and ambient air temperature is 67°F, determine the ratio (%) of efficiency difference between the single glazing collector and the double glazing collector to the efficiency of the single glazing collector. Typical flat-plate solar collector properties U, W/m2C U, Btu/h-t2F Ta No gazing Single glazing Double glazing 0.90 0.85 0.80 2.8 0.5 1.7 0.3 90 00 221

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter7: Forced Convection Inside Tubes And Ducts

Section: Chapter Questions

Problem 7.4P

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1.67 In beauty salons and in homes, a ubiquitous device is the hairdryer. The front end of a typical hairdryer is idealized as a thin-walled cylindrical duct with a 6-cm diameter with a fan at the inlet that blows air over an electric heating coil as schematically shown in the figure. The design of this appliance requires two power settings, with which the air blown over the electric heating coil is heated from the ambient temperature of to an outlet temperature of and with exit air velocities of 1.0 m/s and 1.5 m/s. Estimate the electric power required for the heating coil to meet these conditions, assuming that heat loss from the outside of the dryer duct is neglected.
Heat is transferred at a rate of 0.1 kW through glass wool insulation (density=100kg/m3) with a 5-cm thickness and 2-m2 area. If the hot surface is at 70C, determine the temperature of the cooler surface.
1.60 Two electric resistance heaters with a 20 cm length and a 2 cm diameter are inserted into a well-insulated 40-L tank of water that is initially at 300 K. If each heater dissipates 500 W, what is the time required for bringing the water temperature in the tank to 340 K? State your assumption for your analysis.
A cooling system is to be designed for a food storage warehouse for keeping perishable foods cool prior to transportation to grocery stores. The warehouse has an effective surface area of 1860 m2 exposed to an ambient air temperature of 32C. The warehouse wall insulation (k=0.17W/(mK)) is 7.5 cm thick. Determine the rate at which heat must be removed (W) from the warehouse to maintain the food at 4C.
The hot water needs of a household are to be met by heating water at 15°C to 95°C by a parabolic solar collector at a rate of 1.8 kg/s. Water flows through a 4-cm-diameter thin aluminum tube whose outer surface is blackanodized in order to maximize its solar absorption ability. The centerline of the tube coincides with the focal line of the collector, and a glass sleeve is placed outside the tube to minimize the heat losses. If solar energy is transferred to water at a net rate of 200 W per meter length of the tube, determine the required length of the parabolic collector to meet the hot water requirements of this house. Also, determine the surface temperature of the tube at the exit.
The components of an electronic system dissipating 180 W are located in a 4-ft-long horizontal duct whose cross section is 6 in * 6 in. The components in the duct are cooled by forced air, which enters at 85°F at a rate of 22 cfm and leaves at 100°F. The surfaces of the sheet metal duct are not painted, and thus radiation heat transfer from the outer surfaces is negligible. If the ambient air temperature is 80°F, determine (a) the heat transfer from the outer surfaces of the duct to the ambient air by natural convection and (b) the average temperature of the duct. Evaluate air properties at a film temperature of 100°F and 1 atm pressure. Is this a good assumption?
A thermocouple used to measure the temperature of hot air flowing in a duct whose walls are maintained at Tw = 500 K shows a temperature reading of Tth = 850 K. Assuming the emissivity of thethermocouple junction to be 0.6 and the convection heat transfer coefficient to be h = 60 W/m*• °C. determine the actual temperature of air.
The components of an electronic system dissipating 150 W are located in a 1 -m-long horizontal duct whose cross section is 15 cm X 15 cm. The components in the duct are cooled by forced air, which enters at 30°C at a rate of 0.4 m3/min and leaves at 45°C. The surfaces of the sheet metal duct are not painted, and thus radiation heat transfer from the outer surfaces is negligible. If the ambient air temperature is 25°C, determine (a) the heat transfer from the outer surfaces of the duct to the ambient air by natural convection and (b) the average temperature of the duct.
Solar radiation is incident on the glass cover of asolar collector at a rate of 700 W/m2. The glass transmits88 percent of the incident radiation and has an emissivityof 0.90. The entire hot water needs of a family in summercan be met by two collectors 1.2 m high and 1 m wide. Thetwo collectors are attached to each other on one side so thatthey appear like a single collector 1.2 m * 2 m in size. Thetemperature of the glass cover is measured to be 35°C on aday when the surrounding air temperature is 25°C and thewind is blowing at 30 km/h. The effective sky temperaturefor radiation exchange between the glass cover and the opensky is 240°C. Water enters the tubes attached to the absorberplate at a rate of 1 kg/min. Assuming the back surface of theabsorber plate to be heavily insulated and the only heat lossto occur through the glass cover, determine (a) the total rateof heat loss from the collector, (b) the collector efficiency,which is the ratio of the amount of heat transferred to…
The blades of a wind turbine turn a large shaft at a relatively slow speed. The rotational speed is increased by a gearbox that has an efficiency of 0.93. In turn, the gearbox output shaft drives an electric generator with an efficiency of 0.95. The cylindrical nacelle, which houses the gearbox, generator, and associated equipment, is of length L = 6 m and diameter D = 3 m. If the turbine produces P = 2.5 MW of electrical power, and the air and surroundings temperatures are T = 25 oC and Tsur = 20 oC, respectively, determine the minimum possible operating temperature inside the nacelle. The emissivity of the nacelle is 0.83, and the convective heat transfer coefficient is h = 35 W/m2 .K. The surface of the nacelle that is adjacent to the blade hub can be considered to be adiabatic, and solar irradiation may be neglected. Use Fin or N number of fins to reduce the Ts of the nacelle less than 143 oC
A 3 m internal diameter spherical tank made of 1 cm thick stainless steel (k =15 W/mK) is used to store water at 4℃, as shown in Figure 2.2. Inner surfaceof the tank is 4℃, while the outer surface of the tank is painted with blackcolour paints with emissivity, ? = 0.88 measured at temperature 5℃. It islocated outdoor at 40℃ and is subjected to wind at 25 km/h. Determine the total heat transfer rate to the ice water.
Requires Solution:Determine the power required providing 3 kw of colling at -10 degrees celcius while the surrounding air is at 24 degrees celcius. Answer: 0.39 kW